1. Field of the Invention
The present invention relates, in general, to a bubbler for solid metal-organic precursors and, more particularly, to a bubbler with porous thin plates, improved in feeding efficiency of metal-organic precursors and in precision of controlling the concentration of metal-organic precursors.
2. Description of the Prior Art
In metal-organic chemical vapor deposition (hereinafter referred to as "MOCVD") processes, metal-organic precursors are generally fed in such a manner that they are vaporized at a suitable temperature and then, the vaporized precursors are transferred into a separate reactor by a carrier gas. Herein, the feed rate of the precursors is determined by three parameters, temperature and pressure inside a bubbler and the flow rate of the carrier gas, with the proviso that the contact area between the precursors and the carrier gas should be constant. However, as the reaction time progresses, the contact area is changed in practical bubblers. Hence, it is very difficult to control precisely the feed rate of precursors only with the above three parameters.
In order to better understand the background of the invention, a description for of conventional bubblers will be given below, in connection with some drawings.
Referring initially to FIG. 1A, there is shown a bubbler for metal-organic precursors, comprising a bubbler body 1a, a carrier gas feed tube 1b, and an exhaust tube 1c. As shown in FIG. 1A, bubbler body 1a contains liquid precursor and one end of carrier gas feed tube 1b is dipped in the liquid precursor. On the other hand, exhaust tube 1c is away from the surface of the liquid. FIG. 1B shows the case that the bubbler is charged with a mass of solid metal-organic precursors. After the bubbler is operated for a certain time, there is generated a tunnel 1f, a path through which the carrier gas moves in the mass of solid metal-organic precursors. The width and depth of the tunnel change with the lapse of time, which seriously affects the contact area between a precursor 1e and the carrier gas. Thus, the reaction gas exhausted from exhaust tube 1c carries precursors having a variable composition. Consequently, the conventional bubbler is virtually incapable of precisely controlling the concentration of the precursors in a the reaction gas which is fed into the separate reactor. In addition, the bubbler is disadvantageous in that a good deal of residual precursors 1e is disused because the bubbler should be newly exchanged when the tunnel pierces the precursors in the bubbler.
Referring to FIG. 24, there is shown another bubbler which is made in consideration of the above problems. In contrast with the bubbler shown in FIG. 1A, the bubbler of FIG. 2A has a carrier gas feed tube 2b which is separated from a mass of precursors 2e and an exhaust tube 2c the one end of which is buried in the mass. In this bubbler, a tunnel 2f is also formed but it has a wide width as shown in FIG. 2B and takes a longer time for the tunnel to pierce the mass. Thus, the bubbler of FIG. 2A is improved in duration and consistency of the reaction gas. However, the problems encountered in the bubbler of FIG. 1A cannot be absolutely solved for it is impossible to prevent the formation of the tunnel.
With reference to FIG. 3A, there is shown a further bubbler comprising a bubbler body 3a, a carrier gas feed tube 3b, an exhaust tube 3c and a compressing plate 3d. This bubbler is the same as that of FIG. 2A, except for compressing plate 3d. Likewise, feed tube 3b is located away from a mass of precursors and one end of exhaust tube 3c is buried in the mass. The characteristic element, compressing plate 3d, has many holes and is movable up and down along exhaust tube 3c. Supported by exhaust tube 3c, it is positioned appropriately according to the amount of metal-organic precursor 3e charged. The role of compressing plate 3d is to press precursor 3e by its own weight, expecting that tunnels 3f might be not generated inside the mass of metal organic precursor 3e. However, this bubbler is still defective because the weight of compressing plate 3d is limited and the generation of the tunnel cannot be prevented.